There are many applications for high speed feedback amplifiers which are not slew rate limited. One such application is a transconductance amplifier used in an Ethernet interface chip for tapping into a coaxial cable in an Ethernet system. The interface chip receives a digital signal and converts it to a current in the coax cable. Because of the Ethernet electrical specifications relating to rise time and fall time, frequency components in excess of 10 MHz must be amplified and converted with low distortion to an output current having a range of 0 to 90 milliamps.
Many transconductance amplifiers, while able to pass high frequencies, have stability compromises due to the changes in transconductance, gm, or gain in the output or feedback transistors as a function of the current levels through these transistors. Since the typical Ethernet interface requires variations in output currents of approximately 16 to 1, which results in approximately a four to one change in transconductance in a MOS transistor or a 16 to 1 change in a bipolar transistor, robust compensation techniques are required to provide stability in the interface circuit. However such compensation techniques can impose slew rate limits and cause distortion in the amplified signal. While techniques have been used in the past in amplifiers to detect that an amplifier is in a slew condition and to enable auxiliary circuitry to enhance the slew rate of the amplifier, such enhancement generally do not eliminate the nonlinearities in the output signal of the amplifier.
Therefore, it can be appreciated that compensation circuitry for high speed amplifiers which does not add distortion to the amplifier and which prevents slew rate degradation by the compensation circuitry is highly desirable.